1. Technical Field
The present invention relates to a plate reformer used for producing fuel gas which will be introduced to the anode (fuel electrode) of a fuel cell in a fuel cell power generation system.
2. Background Art
Among the reformers which reform introduced fuel into a hydrogen gas with using catalysts, plate reformers have conventionally been employed as they allow size-reduction and effective reformation by enabling a uniform combustion throughout said combustion chamber.
In the prior reformers, when reforming reaction is performed using natural gas(CH.sub.4) as reforming material gas under the existence of steam (H.sub.2 O), the reforming reaction is: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2
Carbon monoxide and hydrogen are produced as reformed gases during the reaction above. Since this reforming reaction is endothermic, it is necessary to supply heat which is generated in combustion chambers to reforming chamber filled with reforming catalyst for achieving the reaction, and thus plate reformers have been employed wherein reforming chambers and combustion chambers are stacked up one after another.
However, the temperature experienced during the reforming reaction in the reformer is higher than the temperature desirable for operation of molten carbonate fuel cell (600.degree.-650.degree. C.) connected with the reformer, and thus the reforming material gas in the reforming chamber and the combustion gas in the combustion chambers must not flow in the same direction because if they did, the temperature of reformed gas at the exit would reach about 800.degree. C., making direct introduction of the reformed gas to the fuel cell impossible.
Therefore a device has been considered wherein the direction of the gas flow in the reforming chamber is opposite to that of the gas flow in the combustion chamber so that the heat can be retrieved inside the reformer in order to improve heat efficiency of the reformer.
However, simply making the two flows opposite to each other as described above would result in lowered temperature at the exit in both flows (because either gas of which temperature is lower than the other's inevitably interferes and prevents another from achieving high temperature), and the too low temperature at the exit of the reforming chamber would cause part of the reformed gas to be reconverted to methane due to the reverse reaction.
Thus plate reformers wherein the combustion chambers and the reforming chambers have heat exchange sections at both entrance and exit as shown in FIG. 4(a) of the accompanying drawings have been employed.
A plate-shaped reforming chamber 2 filled with reforming catalyst 1 and a plate-shaped combustion chamber 4 filled with combustion catalyst 3 sandwich a heat transfer plate 5 between them, constituting a unit, and two of these units then sandwich a plate-shaped fuel introducing chamber 7 having on both sides the fuel dispersion plates 6 with a number of dispersion holes which allow fuel to flow into the catalyst-filled section of the combustion chambers 4 which are positioned to face each other, thus forming a plate reformer.
Introducing air(or combustion-support gases including oxygen) A to the combustion chamber 4, the fuel F to the fuel introducing chamber 7, the material gas NG for reforming (such as natural gas) and steam S to the reforming chamber 2, causes the fuel F to flow into the combustion chamber 4 through the dispersion holes on the fuel dispersion plate 6, to be combusted by air A therein. The heat generated by the combustion in the combustion chamber 4 is absorbed to the reforming chamber 2 side by way of the heat transfer plate 5, and because of the absorbed heat the material gas NG is then forced to react with the reformation catalyst I inside the reforming chamber 2.
In the plate reformer described above, in order to make heat exchange inside the reformer efficient, the direction of the reformed gas flow in the reforming chamber 2 is made opposite to that of the combusted gas flow in the combustion chamber 4. And also, on both entrance and exit sides of the reaction section X of the reforming chamber 2 filled with the reforming catalyst 1, as well as on both sides of the reaction section X of the combustion chamber 4 filled with the combustion catalyst 3, the heat exchange section Y and Z without catalyst, are provided respectively.
In a conventional fuel cell power generation system, the combustion temperature exceeds the maximum temperature which the combustion catalyst can resist when anode exhaustion gas (emitted from anode) is directly combusted. Therefore, in the plate reformer shown in FIG. 4(a), a fuel introducing chamber 7 is provided wherein fuel F supplied thereto is uniformly dispersed throughout the combustion chamber 4 so that uniform combustion throughout the combustion chamber and thus a temperature of the combustion catalyst which is only slightly higher than the reforming temperature (about 800.degree. C.) can be achieved.
However, providing the heat exchange sections Y, Z on the entrance and exit sides of the reforming and the combustion chambers 2 & 4 increase the size of the system due to the space occupied by the heat exchange section Y and z shown in FIG. 4(a), causing a problem that the size reduction of the whole system cannot be achieved. In addition, since a considerable area is overlapped between the reaction section X of the reforming chamber 2 and the reaction section X of combustion chamber 4, a large discrepancy appears between the locations of the peaks of the temperature distribution profile lI of reforming chamber 2 and the temperature distribution profile I of the combustion chamber 4, thus making it difficult to maintain the maximum temperature of the reforming chamber 2 at the exit as illustrated in FIG. 4(b).
In addition, since higher fuel utilization rate has been proposed recently to achieve higher efficiency of the fuel cell power generation system, concentration of anode gas (fuel gas) contained in the anode exhaustion emitted from the anode of the fuel cell tends to be too thin to supply energy required for reforming to the fuel introducing chamber 3 of the reformer described above.
In order to compensate this shortage, a device has been proposed in the specification of the U.S. Pat. No. 5,208,114, wherein anode exhaustion and cathode exhaustion of a fuel cell are introduced into a catalyst combustor, and then anode gas contained in the anode exhaustion is combusted by air contained in the cathode exhaustion to become a gas with a high temperature which is supplied to the plate reformer so that the sensible heat of the hot gas can be used for increasing the temperature of the reformed gas at the exit.
However, this system requires a catalyst combustor other than a reformer, making the whole system complicated.